Phase varying apparatus for automobile engine

ABSTRACT

PROBLEMS TO BE SOLVED 
     To provide an oil film for permitting transmission of torque between a clutch case and a rotary drum. 
     MEANS FOR SOLVING THE PROBLEMS 
     A phase-varying apparatus for an automobile engine for varying the valve timing of intake and exhaust valves by moving an intermediate member ( 30 ) to cause a relative rotation between an outer cylinder ( 10 ) and an inner cylinder ( 20 ). The apparatus includes a circular rotary drum ( 44 ) connected to the intermediate member ( 30 ) and an electromagnetic clutch ( 42 ) for generating a braking force that acts on the rotary drum  44 . The electromagnetic clutch  42  has an annular clutch case ( 60 ) arranged to face the rotary drum ( 44 ) and an electromagnetic coil  62  for generating the braking force when energized. Grooves ( 66 ) and ( 67 ) are formed in the end face ( 63   a ) of the inner circumferential wall ( 63 ) and in the end face ( 64   a ) of the outer circumferential wall ( 64 ) of the clutch case ( 60 ) such that a torque is transmitted between the rotary drum ( 44 ) and the clutch case ( 60 ) via the oil film formed in and round the grooves ( 66  and  67 ). 
     REPRESENTATIVE FIGURE FIG.  1

TECHNICAL FIELD

This invention relates to a phase-varying apparatus for use with an automobile engine for varying the rotational phase of the camshaft of a valve actuation mechanism relative to its sprocket to thereby change the opening/closing timing of a valve or valves by exerting a braking force of an electromagnetic clutch to the rotary drum operably coupled to the sprocket.

BACKGROUND ART

This type of phase-varying apparatus has been proposed in, for example, Patent Document 1 cited below, in which the camshaft of a valve actuation mechanism is normally rotated in synchronism with a sprocket driven by the crankshaft of the engine via a rotary drum such that when an electromagnetic brake means exerts a braking force to the rotary drum the phase of the camshaft relative to the sprocket is changed.

In this phase-varying apparatus, engine oil is introduced between the frictional faces of the friction member of a clutch case and of the rotary drum in frictional contact with the friction member. The oil is introduced from an oil passage formed in the camshaft to the frictional faces through an oil sump provided inside the clutch case and through an oil feeding cut-away formed in the inner circumferential edge of the front end of the clutch case. As a consequence, these frictional faces are cooled by the engine oil.

Patent Document 1: JPA Laid Open No. 2002-371814 (Pages 4 through 6 and FIGS. 1 through 4)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The phase-varying apparatus disclosed in Patent Document 1 has an electromagnetic clutch constituting a relevant portion of the electromagnetic braking means, which comprises: a generally annular clutch case secured not to rotate about its axis. It has a C-shape transverse cross section and an open end facing the disk face of the rotary drum; an electromagnetic coil accommodated in the clutch case; a plate for holding a friction member (referred to as friction member holding plate) securely fixed to the inside of, and near the open end of, the clutch case; and a flattened frictional member jointed to the friction member holding plate and having a surface slightly projecting from the front end of the inner and outer circumferential walls of the clutch case. The frictional member is made of a fabric that can positively convert the attractive force generated by the electromagnetic clutch into a braking torque that acts on the rotary drum.

However, such fabric friction member is likely to be clogged after a certain period of time and then its friction coefficient μ is significantly reduced. As a consequence, its frequency of use is limited when it is used as the frictional member of the electromagnetic clutch.

It is noted that in order to suppress fluctuations in attractive forces of the electromagnetic clutch a gap (called air gap) of a precise magnitude must be given between the faces of the friction member and the rotary drum by precisely manufacturing the fabric friction member and the rotary drum. However, precision manufacture of an air gap and accurate measurement of the air gap thus formed require special facilities.

It is, therefore, an object of the present invention to overcome such prior art problems as mentioned above by providing a phase-varying apparatus for use with an automobile engine that can transmit a torque from the clutch case to the rotary drum via an oil film lying between them.

Means for Solving the Problems

To achieve the object above, there is provided in accordance with the present invention a phase-varying apparatus for use with an automobile engine, the apparatus including, as defined in claim 1: an outer cylinder subjected to the rotational motion of the crankshaft of the engine; an inner cylinder connected to the camshaft for opening and closing the intake and exhaust valves of the engine and rotatable relative to the outer cylinder; and an intermediate member, disposed between the outer and inner cylinders, for transmitting the rotational motion of the outer cylinder to the inner cylinder, the intermediate member movable in the axial direction thereof to cause a relative rotation of the inner cylinder relative to the outer cylinder to thereby vary the valve timing of the intake and exhaust valves, the phase-varying apparatus characterized in that:

the phase-varying apparatus comprises:

-   -   an annular rotary drum coaxially disposed round the inner         cylinder and connected to the intermediate member, and     -   an electromagnetic clutch for controlling the braking force to         be exerted to the rotary drum in accord with the driving         condition of the engine;

the electromagnetic clutch has

-   -   an annular clutch case arranged facing the rotary drum, and     -   an electromagnetic coil for generating a braking force that acts         on the rotary drum when energized so as to move the clutch case         towards the rotary drum; and

a multiplicity of grooves are formed in at least one of the opposing faces of the rotary drum and the clutch case, for establishing engine oil passages to form an oil film in and around the grooves such that the oil film transmits a torque between the rotary drum and the clutch case.

(Function) When the electromagnetic coil is energized, the clutch case is moved towards the rotary drum, so that the braking force of the electromagnetic coil is exerted to the rotary drum. In this case, because of the oil film staying in and around the grooves formed in at least one of the opposing faces of the rotary drum and the clutch case, a torque is positively transmitted between the clutch case and the rotary drum. As a result, a large friction coefficient μ is obtained without providing any fabric friction member for transmitting a torque between the clutch case and the rotary drum.

As defined in claim 2, the phase-varying apparatus defined in claim 1 may be configured in such a way that

the annular clutch case has an outer and an inner circumferential wall together forming a C-shape transverse cross section and a circular groove between the outer and inner circumferential walls, the circular groove adapted to accommodate therein the electromagnetic coil and facing the rotary drum, and the outer circumferential wall provided on one end thereof facing the rotary drum with the grooves,

grooves are formed in the front end of the outer circumferential wall facing the rotary drum;

a gap is formed between the inner circumferential wall of the clutch case and the rotary drum; and

the rotary drum and the clutch case transmits torque to each other via the oil film formed in and around the grooves.

(Function) Transmission of torque can be performed between the clutch case and the rotary drum by means of an oil film formed in and around the grooves while retaining a gap between the inner circumferential wall and the rotary drum. As a result, the air gap between the inner circumferential wall and the rotary drum can be easily adjusted.

RESULTS OF THE INVENTION

As can be seen from the description given above, a phase-varying apparatus in accord with Claim 1 can provide a large μ without any fabric friction member between the clutch case and the rotary drum.

In a phase-varying apparatus in accord with claim 2, the gap between the inner circumferential wall of the clutch case and the rotary drum can be easily adjusted to adjust the attractive force of the electromagnetic clutch.

FIG. 1 shows a longitudinal cross section of a phase-varying apparatus for use with an automobile engine in accordance with a first embodiment of the invention;

FIG. 2 is a perspective view of the apparatus of FIG. 1, illustrating the internal structure of the apparatus;

FIG. 3 is a front view of the apparatus of FIG. 1;

FIG. 4 is a cross section taken along Line A-A of FIG. 3;

FIG. 5 is a cross section of a relevant portion of the clutch case in accordance with the first embodiment of the invention;

FIG. 6 is an enlarged cross section of the clutch case shown in FIG. 5;

FIG. 7 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a second embodiment of the invention;

FIG. 8 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a third embodiment of the invention;

FIG. 9 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a fourth embodiment of the invention;

FIG. 10 is a front view of a relevant portion of the clutch case in accordance with the fourth embodiment of the invention;

FIG. 11 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a fifth embodiment of the invention; and

FIG. 12 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a sixth embodiment of the invention.

SYMBOLS

-   10 Outer hollow cylinder; 20 Inner hollow cylinder; 30 Intermediate     member; 42 Electromagnetic clutch; 44 Rotary drum; 44 a Disk face;     60 Clutch case; 62 Electromagnetic coil; 63 Inner circumferential     wall; 63 a End face of inner wall; 64 outer circumferential wall; 64     a End face of outer circumferential wall; 66 and 67 Grooves formed     in the end faces;

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described in detail by way of example with reference to the accompanying drawings, in which: FIG. 1 shows a longitudinal cross section of a phase-varying apparatus for use with an automobile engine in accordance with a first embodiment of the invention; FIG. 2 is a perspective view of the apparatus of FIG. 1, illustrating the internal structure of the apparatus; FIG. 3 is a front view of the apparatus of FIG. 1; FIG. 4 is a cross section taken along Line A-A of FIG. 3; FIG. 5 is a cross section of a relevant portion of the clutch case in accordance with the first embodiment of the invention; FIG. 6 is an enlarged cross section of the clutch case shown in FIG. 5; FIG. 7 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a second embodiment of the invention; FIG. 8 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a third embodiment of the invention; FIG. 9 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a fourth embodiment of the invention; FIG. 10 is a front view of a relevant portion of the clutch case in accordance with the fourth embodiment of the invention; FIG. 11 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a fifth embodiment of the invention; and FIG. 12 shows cross sections of relevant portions of the clutch case and rotary drum in accordance with a sixth embodiment of the invention.

Referring to FIGS. 1 and 2, there is shown a phase-varying apparatus of the present invention for use with an automobile engine, which is integrally mounted to the engine. The apparatus is used in an engine oil atmosphere. In this apparatus, the rotation of the crankshaft is transmitted to the camshaft of the apparatus so as to vary the valve timing (that is, opening/closing timing of the intake and exhaust valves) in accord with the load and rpm of the engine.

Specifically, the phase-varying apparatus has: an outer hollow cylinder 10 serving as a sprocket subjected to the driving force of the crankshaft of the engine; an inner hollow cylinder 20 arranged coaxial with, and rotatable relative to, the outer cylinder 10, and constituting a part of the camshaft 2 of the apparatus, an intermediate member 30 arranged between, and in helical spline engagement with, the outer cylinder 10 and inner cylinder 20, the intermediate member is moveable in the axial direction to vary the phase of the inner cylinder 20 relative to the outer cylinder 10; and an electromagnetic brake means 40 provided on one side of the inner cylinder 20 distant from the camshaft 2, for moving the intermediate member 30 in the axial direction. The electromagnetic brake means 40 is mounted on the cover (engine case) 8.

The outer cylinder 10 consists of a sprocket 12 having on the inner circumferential periphery thereof a circular recess 13; an inner flange plate 14 in intimate contact with one side of the sprocket 12 to define flange engagement grooves 13A in collaboration with the circular recess 13; and a spline case 16 being fixed between the sprocket 12 and the inner flange plate 14 by a screw 11 and having a spline engagement section that engages the intermediate member 30.

Provided between a diametrically large recess 13 a formed near the open end of the recess 13 and a diametrically small recess 13 b adjacent the circular recess 13 a is a stepped section 13 c that opposes the outer circumferential edge of a flange 24 formed on the outer circumferential surface of the inner cylinder 20, as described in detail below.

The rotational motion of the crankshaft is transmitted to the outer cylinder 10 (sprocket 12) by a chain C. Flange engagement grooves 13A and a spline engaging section 17 of the outer cylinder 10 can be easily formed by integrating together the sprocket 12, inner flange plate 14, and spline case 16 into the outer cylinder 10 by means of coupling screws.

A male and a female helical splines 32 and 33, respectively, are provided on the respective inner and outer circumferential surfaces of the intermediate member 30. A male helical spline 23 is provided on the outer circumferential surface of the inner cylinder 20. The female helical spline (or spline engaging section) 17 is provided on the inner circumferential surface of the spline case 16. Since the inner and outer splines 32 and 33 of the intermediate member 30 are formed as opposite helical splines that spiral in the opposite directions, only a small axial movement of the intermediate member 30 can cause a large phase shift of the inner cylinder 20 relative to the outer cylinder 10. The intermediate member 30 is provided on the outer circumferential surface thereof with a square male thread 31.

The electromagnetic brake means 40 has an electromagnetic clutch 42 supported by the cover (engine case) 8, a rotary drum 44 rotatably mounted on the inner cylinder 20 via a bearing 22 and engages the square male thread 31 of the intermediate member 30, the rotary drum subjected to the braking force of the electromagnetic clutch 42, and a torsion coil spring 46 disposed between the rotary drum 44 and the outer cylinder 10.

The electromagnetic clutch 42 is mounted on the outer circumferential surface of the boss section 8 a of the cover (engine case) 8. Formed in the inner circumferential surface of the 44 is a female square thread 45 such that the rotary drum 44 and the intermediate member 30 can undergo relative rotations along the female square thread 45 and square male thread 31. In other words, the intermediate member 30 can move in the axial direction while rotationally sliding along the square threads 45 and 31.

The rotary drum 44 and outer cylinder 10 are connected to each other with a twisted torsion coil spring 46 so that when no braking force is exerted to the rotary drum 44, all the outer cylinder 10, inner cylinder 20, intermediate member 30, and rotary drum 44 rotate together. It is noted that the torsion coil spring 46 coaxially extends between the rotary drum 44 and outer cylinder 10 (or spline case 16), so that it contributes to the axial length of the phase-varying apparatus, but contributes to minimization of the radial dimension of the apparatus.

By controlling the electric current supplied to the electromagnetic clutch 42 or turning ON/OFF the electromagnetic clutch 42, the intermediate member 30 can be moved in the axial direction as it rotates along the square threads 45 and 31. This movement will change the phase of inner cylinder 20 relative to the outer cylinder 10, which in turn regulate the opening/closing timing of the valves driven by the cam 2 a of the camshaft 2.

When the electromagnetic clutch 42 is turned OFF (that is, de-energized), the electromagnetic clutch 42 is located at the position shown in FIG. 1 by a phantom line, where a gap S is formed between the rotary drum 44 and electromagnetic clutch 42. In this instance the outer cylinder 10 and inner cylinder 20 are rotated together in phase. As the electromagnetic clutch 42 is turned ON, the electromagnetic clutch 42 is moved to the right to attract the rotary drum 44, thereby exerting a braking force to the rotary drum 44.

This braking force causes the rotational motion of the rotary drum 44 to be retarded relative to the outer cylinder 10. That is, the intermediate member 30 is advanced (or moved to the right in FIG. 1) by the square threads 31 and 45, which in turn causes the inner cylinder 20 (camshaft 2) to be rotated relative to the outer cylinder 10, thereby changing the phase of the camshaft 20 relative to the outer cylinder 10 (sprocket 12). Accordingly, the rotary drum 44 is retained at a position where the braking force balances out the spring force of the torsion coil spring 46, leaving the inner cylinder 20 out of phase by a predetermined angle with respect to the outer cylinder 10.

On the other hand, as the electromagnetic clutch 42 is turned OFF, no braking force acts on the rotary drum 44 any longer, so that the intermediate member 30 is now subjected solely to the spring force of the torsion coil spring 46, which causes the intermediate member 30 to be retracted (or moved to the left as seen in FIG. 1) to its home position. Meanwhile the inner cylinder 20 (camshaft 2) is rotated either in the forward or backward direction with respect to the outer cylinder 10 (sprocket 12), thereby annihilating the phase difference between them.

The circumferential flange 24 is formed on the outer circumferential surface of the inner cylinder 20 journalled in the sprocket 12. On the other hand, the flange engagement groove 13A is formed inside the circumferential inner surface of the outer cylinder 10 (sprocket 12). The flange 24 engages with the groove 13A. Frictional torque enhancement members 51 and 55 are provided between one face of the flange 24 and the engagement face of the grooves 13A. Thus, the frictional torque generated by the outer cylinder 10 in frictional contact with the inner cylinder 20 is increased. At the same time, rattling noises generated by the portions 23, 32, 33, and 17 of the intermediate member 30, outer cylinder 10, and inner cylinder 20 in spline engagement with the square threads 31 and 45 are reduced.

As shown in FIGS. 3-5, the electromagnetic clutch 42 has an annular clutch case 60 having a C-shape transverse cross section with its annular opening facing the disk face 44 a of the rotary drum 44, and having an electromagnetic coil 62 accommodated in the clutch case 60. The electromagnetic coil 62 is fixedly resin-molded within an circular groove 65 formed between the inner circumferential wall 63 and the outer circumferential wall 64 of the clutch case 60.

Provided on, and along the circumference of, the rear face of the clutch case 60 are a multiplicity of projecting pins 68. Each pin 68 engages a corresponding hole 8 b formed in the cover 8. That is, the clutch case 60 is fixedly secured to the cover (engine case) 8 so as not to be rotated about the axis of the camshaft 2 but can slide in the axial direction.

In this case, the end face 63 a of the inner circumferential wall 63 and the end face 64 a of the outer circumferential wall 64 of the clutch case 60 are formed to face the opposing disk face 44 a of the rotary drum 44 such that an oil film of less than 1 μm in thickness is formed between the end face 63 a and disk face 44 a as well as between the end face 64 a and the disk face 44 a.

That is, a multiplicity (90 for example) of oil supplying radial grooves 66 and 67 are formed in the end face 63 a of the inner wall 63 and end face 64 a of the outer circumferential wall 64. In the example shown herein, these radial grooves 66 and 67 are formed at equal angular intervals of 4 degrees in the circumferential directions of the end faces 63 a and 64 a of the inner and outer circumferential walls, respectively.

Each of the grooves 66 and 67 has an arcuate cross section of 0.5 mm in width and 0.15 mm in depth as shown in FIG. 6, and is supplied with oil from an oil sump 74 at all times.

Specifically, as shown in FIG. 1, an oil sump 74 is defined radially inside the clutch case 60 by the cover 8. This oil sump 74 is communicated with an oil passage 70 formed in the camshaft 2 and with the space formed between the clutch case 60 and the rotary drum 44. Pressurized engine oil is pumped by a pump P into the oil passage 70 of the camshaft 2 via the oil port of the journal bearing 73 of the camshaft 2 and the side hole 73 a of the camshaft 2.

The engine oil supplied to the oil passage 70 is introduced into the oil sump 74 via a side hole 73 b formed in the inner cylinder 20. When the electromagnetic clutch 42 is turned off, the engine oil is discharged from the oil sump 74 via a gap between the disk face 44 a of the rotary drum 44 and the clutch case 60, and lead to the front face of the rotary drum 44 via an oil outlet port 80.

On the other hand, when the electromagnetic clutch 42 is energized, the disk face 44 a of the rotary drum 44 and the 60 come closer to each other. As a consequence, the engine oil is discharged from the oil sump 74 via the grooves 66 formed in the end face 63 a of the inner circumferential wall 63 and the groove 67 formed in the end face 64 a of the outer circumferential wall 64, and led to the front face of the rotary drum 44 via the oil outlet port 80.

As a consequence, the disk face 44 a of the rotary drum 44 and the end faces 63 a and 64 a of the clutch case 60 are effectively cooled by the engine oil irrespective of whether the electromagnetic clutch 42 is energized or not.

It is noted that as the engine oil is supplied from the oil sump 74 to the grooves 66 and 67, an oil film is formed between the grooves 66 formed in the end face 63 a of the inner wall 63 and the disk face 44 a of the rotary drum 44, and between the grooves 67 formed in the end face 64 a of the outer circumferential wall 64 and the disk face 44 a of the rotary disk 44.

As a result, torque can be transmitted between the rotary drum 44 and the clutch case 60 via the oil film formed in and around the respective grooves 66 and 67.

In the embodiment described above, a large friction coefficient μarises between the clutch case 60 and rotary drum 44 due to the oil film formed in and around the respective grooves 66 and 67 if no fabric friction member is not provided between the clutch case 60 and rotary drum 44.

The present invention can further achieve the following results:

(1) Reduction of the friction coefficient μ due to clogging of the friction member can be circumvented, since no friction member is mounted in the radial grooves 65 of the clutch case 60; (2) The apparatus requires a less number of components, and hence can be manufactured with less cost, since no friction member is needed; and (3) Frictional wear of the clutch case 60 can be reduced even when the rotary drum 44 is made of an alloy if the clutch case 60 is made of a soft steel, provided that the clutch case 60 and the rotary drum 44 are made to have sufficiently different surface hardnesses.

Referring to FIG. 7, there is shown a second embodiment of the invention. In this embodiment, there is provided an space (hereinafter referred to as air gap (AG)) between the end face 63 a of the inner wall 63 of the clutch case 60 and the disk face 44 a of the rotary disk 44. Rest of the features of this embodiment are the same as those of the first embodiment.

In this embodiment, it is possible to achieve a large friction coefficient μ between the clutch case 60 and rotary drum 44 without mounting a fabric friction member between them owing to the oil film formed in and around the grooves 67.

This embodiment can achieve the same results as the first embodiment. Furthermore, by decreasing the initial metallic conformity of the rotary drum 44 with the clutch case 60, the air gap between the rotary drum 44 and clutch case 60 can be decreased to enhance the attractive force of the electromagnetic clutch, thereby compensating for the so-called initial reduction in μ.

In this configuration, the rigidity of the clutch case 60 can be sufficiently increased, which enables regulation and measurement of the air gap under a stable condition.

Next, a third embodiment will be described with reference to FIG. 8. In this embodiment the end face 64 a of the outer circumferential wall 64 of the clutch case 60 is tapered and is formed with grooves 67 (not shown). The rest of the features of the third embodiment are the same as those of the second embodiment.

The third embodiment can achieve the same results as the second embodiment, since transmission of torque between the rotary drum 44 and clutch case 60 is performed via the oil film formed in and round the respective grooves 67.

Next, the fourth embodiment of the invention will now be described with reference to FIGS. 9 and 10. In this embodiment, the end face 64 a of the outer circumferential wall 64 of the clutch case 60 is provided with grooves 69 each formed oblique to the radius perpendicular to the axis of the inner cylinder 20 and passing through the groove.

In this embodiment, since transmission of torque between the rotary drum 44 and clutch case 60 is performed via the oil film formed in and around the respective grooves 67 as in the second embodiment, the same results can be achieved as in the second embodiment.

Referring to FIG. 11, there is shown a fifth embodiment of the invention. In this embodiment, the open end of the outer circumferential wall 64 of the clutch case 60 has a two-step section 76. The end face 76 a of the two-step section 76 has grooves 67 (not shown) serving as oil-flow passages.

In this embodiment also transmission of torque between the rotary drum 44 and clutch case 60 is performed via the oil film formed in and around the respective grooves 67 as in the second embodiment, so that the same results can be achieved as in the second embodiment.

The configuration of the open end of the outer circumferential wall 64 of the clutch case 60 that it has the two-step section 76 and the end face 76 a has grooves 67 (not shown), allows a larger degree of freedom in the design of the two-step section 76 having a larger width in the radial direction, which provides better controllability of the attractive force of the electromagnetic clutch.

Referring to FIG. 12, there is shown a sixth embodiment of the invention in which an annular tapered section 77 is formed at the open end of the outer circumferential wall 64 of the clutch case 60 and an opposing annular tapered section 78 is formed on the disk face 44 a of the rotary drum 44, so that oil flow passages are provided in the form of the grooves 67 (not shown) in the surface of the tapered section 77. The rest of the features of this embodiment are the same as those of the second embodiment.

In this embodiment also, transmission of torque between the rotary drum 44 and clutch case 60 is performed via the oil film formed in and around the respective grooves 67 as in the second embodiment, so that the same results can be achieved as in the second embodiment.

Moreover, since torque is transmitted between the rotary drum 44 and clutch case 60 via the oil film formed between the annular tapered sections 77 and 78, the attractive force of the electromagnetic clutch 42 is enhanced in the axial direction, and so is the torque.

Although the invention has been described and shown in the examples above as having grooves 66, 67, and 69 in the clutch case 60, a multiplicity of grooves may be formed alternatively in the disk face 44 a of the rotary drum 44, thereby forming an oil film in and around the grooves for transmitting torque between the rotary drum 44 and clutch case 60. 

1. A phase-varying apparatus for use with an automobile engine, including: an outer cylinder subjected to the rotational motion of the crankshaft of the engine; an inner cylinder connected to the camshaft for opening and closing the intake and exhaust valves of the engine and rotatable relative to the outer cylinder; and an intermediate member, disposed between the outer and inner cylinders, for transmitting the rotational motion of the outer cylinder to the inner cylinder, the intermediate member moveable in the axial direction thereof to cause a relative rotation of inner cylinder relative to the outer cylinder to thereby vary the valve timing of the intake and exhaust valves, the phase-varying apparatus characterized in that: the phase-varying apparatus comprises: an annular rotary drum coaxially disposed around the inner cylinder and connected to the intermediate member, and an electromagnetic clutch for controlling the braking force to be exerted to the rotary drum in accord with the driving condition of the engine; the electromagnetic clutch has an annular clutch case arranged to face the rotary drum, and an electromagnetic coil for generating a braking force that acts on the rotary drum when energized so as to move the clutch case towards the rotary drum; and a multiplicity of grooves are formed in at least one of the opposing faces of the rotary drum and the clutch case, for establishing engine oil passages to form an oil film in and around the grooves in such a way that the oil film transmits a torque between the rotary drum and the clutch case.
 2. The phase-varying apparatus according to claim 1, wherein: the annular clutch case has an outer and an inner circumferential wall together forming a C-shape transverse cross section and a circular groove between the outer and inner circumferential walls, the circular groove adapted to accommodate therein the electromagnetic coil and facing the rotary drum, the grooves are formed in one end of the outer circumferential wall facing the rotary drum; a gap is formed between the inner circumferential wall of the clutch case and the rotary drum; and the rotary drum and the clutch case transmits torque to each other via the oil film formed in and around the grooves. 